<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            width: 300px;
            height: 300px;
            border: 1px solid #000; 
        }
        .red {
            background-color: #f00;
        }
        
    </style>
</head>
<body>
    <div id="app">
        <button @click="isRed = !isRed">切换</button>

        <!-- 
            当isRed为true就有red这个类
            当isRed为false就没有red这个类
         -->
        <div class="box" :class="{ red: isRed }"></div>
      </div>
    <script src="./vue.js"></script>
    <script>
        new Vue({
            el: "#app",
            data() {
                return {
                   isRed: true
                }
            },
            methods:{
                   
            }
        })
    </script>
</body>
</html>